1. Field of the Invention
The present invention relates to a method for packet retransmission, and more particularly, to a method for packet retransmission employing feedback information, in which a transmitter changes a retransmission mode and retransmits packets in accordance with information fed back from a receiver to enhance decoding probability of received packets.
2. Background Art
A communication system for next generation is designed to use a broadband frequency bandwidth and increase a data rate between a transmitter and a receiver. For configuration of such a communication system, an OFDM based modulation mode is mainly adopted. Examples of the wireless communication standard which uses the OFDM based modulation mode include 3GPP LTE(+), 3GPP2 UMB(+), IEEE 802.16(d,e,m), IEEE 802.11, IEEE 802.20, IEEE 802.22, and HiperLAN. Examples of user classification modes in the communication system include FDMA mode in which users are classified based on a frequency axis and a TDMA mode in which users are classified based on a time axis. In the communication standard configured at the low cast, such as wireless LAN and HyperLAN, CSMA corresponding to the TDMA mode is mainly used. On the other hand, in the system such as 3GPP LTE, 3GPP2 UMB, and IEEE 802.16, which supports commercial voice communication, traffics of respective user equipments are classified from one another in accordance with an FDMA mode to maximize frequency efficiency. Accordingly, the above systems allow data to be transmitted under the control of a base station, and information for the control is transferred to a user equipment through scheduling information.
In a communication procedure between a user equipment and a base station, scheduling for maximizing frequency efficiency and obtaining multi-user diversity is mainly used. In other words, frequency resources are allocated in such a manner that a specific user equipment obtains maximum throughput using minimum frequency resources. The scheduling result according to the above allocation is set to be decoded by the user equipment without error and then transferred to the base station.
However, if channel status of the user equipment is not good, packet transmission between the base station and the user equipment does not end only one time but is accompanied with retransmission. Time-diversity and transmission energy boosting effects can simultaneously be obtained by packet retransmission. Although packet retransmission reduces maximum throughput, it is suitable to configure a reliable channel. A hybrid automatic repeat request (HARQ) mode is mainly used for initial transmission/retransmission of packets. Packets transmitted and received between the user equipment and the base station are encoded through a channel code. Various HARQ modes are generated in accordance with a method of converting packets in various formats. The simplest retransmission mode is that all codewords are used for every packet transmission. A receiver generally performs decoding using chase combining. As another method, there is provided an incremental redundancy (IR) method of transmitting codewords by splitting them. If new code bit information is received as retransmitted packets, it means that additional information is received. If the existing bits are received as retransmitted packets, bit combining is performed.
FIG. 1 illustrates that configuration of transmission packets may be varied whenever the packets are transmitted through HARQ.
If chase combining is used, retransmitted packets are equally configured. However, if IR is used, configuration of packets may be varied whenever the packets are retransmitted.
FIG. 2 illustrates a decoding procedure in a receiver.
It is assumed that IR is used. In this case, if the receiver receives retransmission packets, the receiver sets a part of the packets which are not received in each codeword to reliability 0 and performs decoding, i.e., soft decision decoding. Alternatively, the receiver assumes the part of the packets which are not received as erasure and then performs decoding, i.e., hard decision decoding.
Also, in case of codeword bits received in a multi-mode, the received bits are used by combination. In this case, the bits may be combined with one another in accordance with soft decision or hard decision. When the bits are combined with one another in accordance with soft decision, it is assumed that channel correction has been performed. Then, the receiver combines the received signals with one another and averages them through analog type or multi-bit precision. In this case, weighted combining can be performed in accordance with accuracy of the received bits. On the other hand, if only bits of 0 and 1 are required like hard decision decoding, 0 and 1 are selected based on majority when the respective bits are combined with one another.
The following Equation 1 represents soft combination when soft decision is performed, and the following Equation 2 represents majority selection when hard decision is performed. The actual configuration of each of Equations 1 and 2 may be varied depending on a decoding method.
                              R          ⁡                      (            k            )                          =                              ∑                          i              =              1                                      N              R                                ⁢                                    w              ⁡                              (                                  k                  ,                  i                                )                                      ⁢                          r              ⁡                              (                                  k                  ,                  i                                )                                                                        [                  Equation          ⁢                                          ⁢          1                ]                                          R          ⁡                      (            k            )                          =                  {                                                                      1                  ,                                                                                                                        ∑                                              i                        =                        1                                                                    N                        R                                                              ⁢                                                                  w                        ⁡                                                  (                                                      k                            ,                            i                                                    )                                                                    ⁢                                              r                        ⁡                                                  (                                                      k                            ,                            i                                                    )                                                                                                      >                                                            N                      R                                        2                                                                                                                        0                  ,                                                            Otherwise                                                                        [                  Equation          ⁢                                          ⁢          2                ]            
In this case, R(k) means soft/hard decision information of the received bits, r(k,i) means soft/hard decision value in the ith HARQ packet reception, and w(k,i) means a weight value to be applied to a corresponding bit in each transmission packet when combining is performed.
The aforementioned transmission/decoding method is suitable for a general blind channel. Also, the channel can have optimal throughput in a state that all bits have the same uncertainty as one another, like AWGN. Actually, most of wireless channels have frequency selective feature. For this reason, a problem occurs in that there is no ideal aspect in view of conventional IR or chase combining. In order to solve this problem, if blindness of channel is maintained, interleaver can be changed or subpackets of IR can be selected supposing mutual independent subcarrier features.
However, in order to obtain diversity of actual channel, additional channel information bits are provided to obtain better throughput.
If resources are allocated to a whole system bandwidth for packet transmission in a state that the transmitter does not know channel information, i.e., in case of a resource mode beyond a coherence bandwidth with a localized allocation mode, the transmitter needs to improve its transmission method to allow the receiver to use channel bandwidth more effectively. In this case, the number of times for packet retransmission is reduced, and packet transmission latency can be reduced.
However, in a state that the transmitter does not know channel information, there is limitation in obtaining diversity of actual channel.